1. Field of the Invention
This invention relates to the production of sensory stimulation through the exposure of neural tissue to electromagnetic fields, and producing sensory anesthesia.
2. Description of the Prior Art
At the present time, all methods of sensory stimulation involve stimulation of the end organ. It will be understood that reference to sensory stimulation in this patent application refers to stimulation of the sensory nerves for the senses of smell, taste, and touch. For example olfactory stimulation, senses of smell, is achieved with the chemicals of scent. Perfume, air freshener scented soaps and candles are examples of the means by which a consumer experiences scent.
Nerve impulses are transmitted in the body by the nervous system which includes the brain, spinal cord, nerves, ganglia and the receptor. Nerves are made up of axons and cell bodies together with their respective protective and supporting structures. The axon is the long extension of the nerve cell that conducts nerve impulses to the next neuron.
The propagation of the nerve impulse along the axon is associated with an electric potential and a flow of cations into and out of the axon. This electric potential is called the action potential. The typical human action potential has an advancing front of depolarization with a peak value of +40 mV. In order to continue to propagate, the action potential must trigger the depolarization of the neural tissue directly at the front of the advancing wave.
In order to produce depolarization, the interior of the axon must be depolarized from its resting potential of xe2x88x9270 mV (a typical resting voltage potential) to a potential of xe2x88x9260 mV. However, once xe2x88x9260 mV is reached, the sodium channels in the axon opens and causes sodium cations (Na+) to flow into the axon, thereby allowing the depolarization to proceed to +40 mV. Other ion channels then open and cause the potassium cations in the axon to flow out of the axon until the interior of the cell repolarizes to xe2x88x9270 mV.
Thus, all that is necessary to propagate the action potential is to have an external potential which can bring the interior of the cell to xe2x88x9260 mV. Since the action potential consists of an advancing wave of +40 mV, under normal conditions the interior of the cell will depolarize to xe2x88x9230 mV (xe2x88x9270 mV+40 Mv) which is more than enough to propagate the action potential. As is known, these potentials are externally induced transmembrane potentials which are measured across the wall of the axon.
Given the fact that the nerve impulse is transmitted along the axon due to an electrical potential (the action potential), there have been a number of studies into the artificial propagation of nerve impulses using various electrical devices. For example, electrodes have been inserted into a nerve and a current passed through the nerve to cause movement of muscles.
Direct application of electric current has also been used to effect neurostimulation. In this technique, electrodes were applied directly to the skin or to underlying structures in a way which created an electric current between the two electrodes in the tissue in which the target neuronal structure was located. This technique employed a constant voltage source and was intended to cause neuronal transmission and thereby produce stimulation both in peripheral nerves and in the brain.
The influence of an external electric field on neuronal tissue has also been studied. One model for this is the effect of a monopolar electrode in the proximity of a neuron. (Rattay F, J Theor. Biol (1987) 125,339-349). The model for electrical conduction in the neuron which has been widely accepted is the modified cable equation:                               ∂                      xe2x80x83                    ⁢                      V                          ∂              t                                      =                              [                                                            d                                      4                    ⁢                                          ρ                      i                                                                      ⁢                                  (                                                            ∂                                              V                                                  ∂                                                      x                            2                                                                                                                +                                          ∂                                                                        V                          e                                                                          ∂                                                      x                            2                                                                                                                                )                                            -                              i                i                                      ]                    /                      c            m                                              (                  eqn          ⁢                      xe2x80x83                    ⁢          1                )            
where:
V represents voltage,
ii is the total ionic current density,
xcfx81i=the resistivity of the axoplasm,
cm=capacitance of the membrane,
Ve=externally applied voltage,
the term:                     ∂                              V            e                                ∂                          X              2                                                          (                  eqn          ⁢                      xe2x80x83                    ⁢          2                )            
xe2x80x83is referred to the activating function by Rattay because it is responsible for activating an axon by external electrodes.
The activating function has two possible effects on an axon. If its magnitude is sufficient there is a superthreshold response. This leads to the generation of an action potential. If this occurs then the cable equation will predict the expected response. In order to calculate the equation however the ionic current term ii must be calculated.
In order to calculate the ionic current, an equation of membrane ionic current, as a function of the externally induced transmembrane potential, is used. For myelinated membranes the Hodgkin-Huxley equation can be used. For unmyelinated membranes the Huxley-Frankenhaeuser equation is used.
There are other equations which account for membrane temperature as well. The other possible effect on the axon is subthreshold stimulation.
If a subthreshold stimulus is applied, then the transmembrane voltage is directly related to the activating function. The voltage changes due the opening of voltage sensitive ionic channels can be ignored. The calculation of transmembrane voltage becomes simplified. It is the subthreshold stimulation of the neuron which is considered in this patent application.
The present invention produces sensory stimulation by exposing a patient to different spatially and temporally varying electromagnetic fields by means of a magnetic flux generator positioned external to the patient. More specifically, the invention produces areas of depolarization which lead to the propagation of nerve signals. Thus, this invention provides for an entirely new form of sensory stimulation. In addition to sensory stimulation this invention provides a means to block retrograde neural conduction that arises when sensory stimulation is produced. This same method of neural blockade can be used on its own to produce sensory anesthesia.
The term xe2x80x9celectromagnetic sensory stimulationxe2x80x9d will be used to refer to stimulating nerves with a varying electromagnetic field using a magnetic flux generator positioned external to a body in this specification. It also produces areas of hyperpolarization which act to prevent the transmission of action potentials down axons surrounding the target axon. Thus the axonal stimulation can be focused by using a combination of depolarization and hyperpolarization.
This invention also provides a localizing system. In the prior art of neurostimulation, it is usually administered in a manual way in which the neurophysiologist places a needle in close proximity to the targeted nerve by direct hand manipulation. The needle is manipulated to the endpoint of eliciting paresthesia or by muscle twitch when using a neurostimulator.
However, with manual needle manipulation, it is difficult to guarantee smooth progression from one point to another. It is also difficult to ensure that all points within a certain region have been probed with the needle. The present invention avoids these problems.
Broadly, the method of the present invention is a method for sensory stimulation or sensory blockade in a patient comprising the steps of creating a time varying magnetic field with a device positioned completely external to said patient, said time varying magnetic field resulting in an electric field which creates one or more regions of hyperpolarization or depolarization along neural tissue, for as long as required, said regions of depolarization causing the propagation of a sensory neural impulse and said regions of hyperpolarization being of sufficient magnitude to block the propagation of nerve impulses in said neural tissue preventing retrograde sensory conduction or producing sensory anesthesia.
Preferably, the device is a coil which can produce a time varying magnetic field. Also, preferably, the device consists of a resistor, capacitor and inductor in series. The capacitor is discharged through the device so as to form a time varying magnetic field which in turn creates said electric field.
Preferably, the coil is circular in shape and has about 7 to about 10 turns, and the coil has a diameter of about 3 to about 7 cm.
Preferably, a time varying current passes through the device and the time varying current increases from 0 to about 6000 amps in 60 microseconds.
It is also preferred that the coil has a resistance (R) of about 0.1 to about 0.5 ohms and an inductance (L) of about 10 to about 90 microhenries.
The present invention also includes a system which consists of multiple devices as described above and each of the devices is configured so that the effect of the devices is to produce a continuous blockade of one or more nerves. It is also preferred that the system produces an electric field which creates a triphasic potential within the axon, said triphasic potential consisting of a virtual cathode surrounded by a virtual anode on each side of said virtual cathode.
The present invention also comprises a configuration consisting of multiples of the system which permit blockades of more than one neuron at the same time.
The system of the present invention preferably has coils of wire which are organized in such a way to allow production of a strong focused electric field at one or more interior points in the brain while sparing all other points in the brain.
Preferably, the coils are positioned with a three dimensional electromechanically controlled positioning system. It is also preferred that the coils are affixed to the body part containing a target nerve thus providing for continuous acute or chronic pain control.
The present invention can further be characterized as a method for producing sensory stimulation or sensory anesthesia in a patient by:
(1) creating a magnetic flux with a magnetic flux generator, said generator being positioned completely external to the patient and not in physical contact with the patient; and
(2) treating a nerve of said patient with said magnetic flux to cause a depolarized region, a hyperpolarized region, or a combination of depolarized and hyperpolarized regions along an axon which leads to a focused propagation of an action potential in said nerve.
As is known, a time varying magnetic field, which is a magnetic flux, results in an electric field. The orientation and strength of the magnetic flux and its resulting electric field is such that it depolarizes, hyperpolarizes or both depolarizes and hyperpolarizes regions along the axon so as create areas of neurostimulation and regions of neuronal blockade( which prevents the propagation of the action potential).
To accomplish sensory stimulation in accordance with the present invention, a preliminary study is performed wherein the sensory nerve response to specific stimuli is recorded. More specific microelectrodes are used to measure the individual axonal response of a nerve to specific sensory stimuli. The axonal action potentials created by the magnetic flux reproduce the previously measured axonal responses to sensory stimuli thereby causing sensory perception. Those areas of hyperpolarization prevent the action potentials from traveling in the retrograde direction (opposite the normal direction of flow). These areas of blockade are used to cause sensory anesthesia or analgesia.
The strength of depolarization/hyperpolarization is expressed as a voltage and is a measurement of the voltage or electric potential between the inside of the axon and the outside of the axon. This electric potential is sometimes referred to as the externally induced transmembrane potential since it is measured across the cell wall.
For depolarization, the magnetic flux should be of such an orientation and strength so as to create a net externally induced transmembrane potential equal to or greater than xe2x88x9260 mV. More preferably, the electric potential created by the magnetic flux of the present invention should be about xe2x88x9250 mV or greater and, more preferred, about xe2x88x9240 mV or greater. It should be understood that xe2x80x9cgreaterxe2x80x9d means more positive.
For hyperpolarization, the magnetic flux should be of such an orientation and strength so as to create an electric potential of less than xe2x88x92100 mV. More preferably, the electric potential created by the magnetic flux should be about xe2x88x92110 mV or less, and, more preferably, about xe2x88x92120 mV or less. It should be understood that xe2x80x9clessxe2x80x9d means increasing in negativity.
The orientation of the electric field is such that it has a component parallel to the long axis of the axon. It is thought that the depolarization and/or hyperpolarization prevents the sodium and potassium gates from moving the cation across the cell membrane.
Preferably, the configuration of magnetic flux generators produces both a depolarized region and a hyperpolarized region adjacent to each other. This ensures only one way propagation of a nerve signal down an axon.
The magnetic flux generator of the present invention, consists of a RLC (resistor, inductor capacitor) circuit with a coil of wire as the inductor. The magnetic flux generates both a depolarized and hyperpolarized region. The externally induced transmembrane potentials for the combined depolarized and hyperpolarized regions have a strength and orientation as referred to above, e.g. depolarized equal to or greater than xe2x88x9260 mV, and hyperpolarized less than xe2x88x92100 mV.
Suitable magnetic flux generators which can be used in the present invention include any device which is capable of creating and projecting a magnetic flux. It is known that an antenna with associate circuitry can create and project magnetic fluxes. Additionally, a magnetron tube with associate circuitry is also capable of creating and projecting magnetic fluxes. Preferably, a circuit with an inductive element is used.
Suitable inductive elements include a coil of wire of various shapes and sized such as a round, figure eight, square, torroidal, etc. Most preferably, the magnetic flux generator is an RLC circuit with a round coil of wire. As will be appreciated by one of skill in the art, any high DC voltage pulsed power supply can be employed with the inductive elements.
The round coil of wire is preferably in series with a capacitor and a resistor so as to form a RLC circuit. The capacitor is discharged through the device so as to form a time varying magnetic field which in turn creates an electric field.
Additionally, two side-by-side magnetic flux generators can be used so as to create a distal hyperpolarized region and a proximal depolarized region. In order to create a unidirectional neural impulse the requisite axonal externally induced transmembrane potentials for these side by side magnetic field generators is the same as for the individual coil case. That is the depolarized region must be equal to or greater than xe2x88x9260 mV and the adjacent hyperpolarized regions are less than xe2x88x92100 mV.
Furthermore, a plurality of magnetic flux generators can be used such that no one individual magnetic flux generator produces the necessary electric field but that the combined generators, when oriented towards the same axon, produces a net electric field. That net electric field is of sufficient magnitude to produce a hyperpolarized and depolarized region of sufficient externally induced transmembrane potential to produce a unidirectional neural impulse. This technique of a focused array of coils would be used for dense neural tissue such as the spinal cord or brain where the field effects must be localized to a small region.
Another means of producing a focused electric field is to displace the magnetic coil so that only the relevant portion of the electric field is exposed to the nerve. This will also produce a focused externally induced transmembrane potential change. Putting two coils in the proper displaced orientation can produced two focused regions. One focused region of depolarization and one of hyperpolarization.